Oh my. “buster” is alphabetically later than “bullseye”. Not sure out of my head what the best way would be to solve this issue in package versioning, but one can probably find hints in Debian policy/manuals. @Checkmk_tribe might want to look into these.
You could uninstall the buster package, but to be able to do so you would have to temporarily remove the “version” symlink in your site directory.
Instead I would try to install with apt instead of gdebi and see if that already helps. Or even with dpkg --install check-mk-enterprise-2.0.0p17_0.bullseye_amd64.deb (and clean up failed dependencies later with apt-get -f install without any package name).